Planar decimate style in Cycles Shader?
I've done this with a baked normal map, but as you point out, image textures do require a bit of memory. I've already prepared two characters, one is the default character from MakeHuman with two subdivisions, and the other is a duplicate of that, which I decimated with Planar and an angle of 5 degrees. I've also made sure they're aligned with each other on all axes.  

Prepare an image to contain the normal map. Open up the UV/Image Editor and click New. I used the default settings.
  

Select the undecimated character and go into the Compositor. Add an image texture and change it to Non-Color Data. Add a Texture Coordiante node and connect its UV socket to the Vector socket of the Image Texture node. Do not connect the output of the Image Texture node to anything yet. If you do, you may get a Circular Reference error when baking.
  

Go back to the 3D View.Expand the Bake section in the Render panel, change Bake Type to Normal, enable Selected to Active, and set the Ray Distance to something higher than 0. Select the two objects. Make sure you select the decimated one first, then while holding Shift select the undecimated one. Then click Bake.
  

Once the baking is done, open up the UV/Image Editor and select the image you created. It should now contain the normal map. Make sure you save it or pack it before doing anything else, or you may lose it.
  

Go back to the 3D View and select the undecimated object, the go into the Compositor. Add a Normal Map node. Connect the output from the Image Texture node to the Color input of the Normal Map node. The connect its Normal output to the Normal input of any shader you want to be affected by this.
  

Here's the result I got with my characters. I've exaggerated the effect of the normal map by turning its strength up to 2.
  

With this render, RAM usage peaked at 63.95MB. I'm not GPU rendering, because my GPU is basically a potato, and I do not know if textures require more, less or the same amout of RAM when rendering on GPU. I used default settings when creating the image, so I did not enable the 32 bit Float option and the size is 1024x1024. If you require higher quality, it will obviously use up more RAM.